The invention relates to a method of forming a desired optical image having a given minimum feature size on a selected surface of material sensitive to optical radiation, the method comprising the steps of:
providing a source of optical radiation having at least one predetermined wavelength xcex;
providing a layer of material, sensitive to optical radiation having the wavelength xcex, with a selected surface to receive an optical image thereon;
positioning a mask, comprising image information and being at least partly transparent to the source radiation, between the radiation source and the layer of sensitive material, so that source radiation is transmitted through the mask towards the sensitive layer, and
illuminating the selected surface with source radiation received by transmission through the mask to produce the desired optical image.
The invention also relates to a mask for use with this method, a method of manufacturing a device using this method and an apparatus for carrying out this method.
This method and apparatus, referred to as proximity printing method and apparatus, are used, inter alia, in the manufacture of liquid crystalline display (LCD) panels, customer ICs (integrated circuits) and PCBs (printed circuit board). Proximity printing is a fast and cheap method of forming an image, comprising features corresponding to the device features to be configured in a layer of a substrate, in a radiation-sensitive layer on the substrate. Use is made of a large photo mask that is arranged at a short distance, called the proximity gap, from the substrate and the substrate is illuminated via the photo mask by, for example, ultraviolet (UV) radiation. An important advantage of the method is the large image field, so that large device patterns can be imaged in one image step. The pattern of a conventional photo mask for proximity printing is a true, one-to-one copy, of the image required on the substrate, i.e. each picture element (pixel) of this image is identical to the corresponding pixel in the mask pattern.
Proximity printing has a limited resolution, i.e. the ability to reproduce the points, lines etc., in general the features, in the mask pattern as separate entities in the sensitive layer on the substrate. This is due to the diffractive effects which occur when the dimensions of the features decrease with respect to the wavelength of the radiation used for imaging. For example, for a proximity gap of 100 xcexcm, the resolution is 10 xcexcm, which means that pattern features at a mutual distance of 10 xcexcm can be imaged as separate elements. To increase the resolution in optical lithography, real projection apparatus, i.e. apparatus having a real projection system like a lens projection system or a mirror projection system, are used. Examples of such apparatus are wafer steppers or wafer step-and scanners. In a wafer stepper, a mask pattern, for example an IC pattern is imaged in one step by a projection lens system on a first IC area of the substrate. Then the mask and substrate are moved relative to each other until a second IC area is positioned below the projection lens. The mask pattern is then imaged on the second IC area. These steps are repeated until all IC areas of the substrate are provided with an image of the mask pattern. This is a time consuming process, due to the sub-steps of moving, aligning and illumination. In a step-and-scanner, only a small portion of the mask pattern is illuminated and, during illumination, the mask and the substrate are synchronously moved with respect to the illumination beam until the whole mask pattern has been illuminated and a complete image of this pattern has been formed on an IC area of the substrate. Then the mask and substrate are moved relative to each other until the next IC area is positioned under the projection lens and the mask pattern is scan-illuminated, so that a complete image of the mask pattern is formed on the next IC area. These steps are repeated until all IC areas of the substrate are provided with a complete image of the mask pattern. The step-and-scanning process is even more time consuming than the stepping process.
If a 1:1 stepper, i.e. a stepper with a magnification of one, is used to print an LCD pattern, a resolution of 3 xcexcm can be obtained, however at the expense of much time for imaging. Moreover, as the image field is divided into sub-fields, stitching problems may occur, which means that neighbouring sub-fields do not fit exactly together.
It is an object of the present invention to provide a proximity printing method and a mask for use therewith which allow increase of the resolution of the order of that obtainable with an optical lithographic projection apparatus for the same purpose, and, moreover, opens the way to new possibilities which cannot be realized by either conventional proximity printing methods or steppers or step-and-scanners. The method is characterized in that use is made of a mask in the form of a diffractive element wherein the image information is encoded in a two-dimensional array of image cells having dimensions which are smaller than the minimum feature size, each image cell having one out of at least two specific transmission levels and one out of at least three phase levels.
The amplitude level and the phase level of an image cell are measures of the degree to which the amplitude and the phase, respectively, of a beam portion incident on this image element are changed by the image cell.
The method of the invention makes effective use of the diffraction effect, which is the resolution limiting factor in the conventional method. The mask pattern is no longer identical to the required image on a pixel by pixel basis, but is encoded in small image cells. Such an image cell is not identical to a given pixel in the required image, but changes, in co-operation with a number of neighbouring image cells, both the amplitude and phase of the portion of the illumination beam passing through the area of these image cells, such that a portion of the required image is formed in the sensitive layer on the substrate. The method uses two independent coding parameters: the amplitude and the phase of an image cell, which enlarges the mask design possibilities. Each of these parameters has one out of a number of discrete values, or levels. The number of amplitude levels is two or more and the number of phase levels is three or more. The mask design is the result of a computing technique similar to the technique of computer generated holograms. The resulting amplitude level for each image element may be implemented in the mask by a coat layer, having a specific transmission, on the image information carrying surface of the transparent mask substrate and the phase level by adapting the thickness of this substrate.
It is noted that the article: xe2x80x9cX-ray holography for VLSI using synthetic bilevel hologramsxe2x80x9d in SPIE, Vol. 3183, 1997, pages 2-13 describes a specific X-ray proximity printing method which uses synchroton radiation of very short wavelength, of the order of 1 nm, synchrotron radiation for the manufacture of VLSI electronic circuits having smallest feature sizes of the order of 100 nm. To increase the gap width from the unpractical value of 4 xcexcm to the more practical value of 10 xcexcm or larger, the conventional X-ray mask, having a pattern configuration identical to the configuration of the image to be formed, is replaced by a so-called bilevel computed hologram. The known method differs from that of the present invention not only in that the ratios of the smallest feature size and the wavelength used and the gap width, respectively are totally different, but also in that the mask elements are several hundred X-ray wavelengths thick, so that waveguide effects play a role. Moreover, the hologram used in the X-ray method has only two phase levels, which introduce a phase shift of zero and xcfx80 radians, respectively.
Also U.S. Pat. No. 5,612,986 describes a method for imaging an IC mask pattern on a substrate via a mask and by means of X-ray radiation. The conventional X-ray mask has been replaced by a diffraction element having several amplitude steps and several phase steps. However, the amplitude steps are the result of the different thicknesses of the mask substrate needed for the phase steps and the different amplitude attenuation introduced thereby. The phase level and amplitude level of an image cell cannot be chosen independently of each other.
The method of the invention is preferably characterized in that the illumination step comprises illuminating the mask with a beam having an aperture angle of the order of a few degrees.
The aperture angle of the illumination beam is understood to be the angle between the outermost rays of this beam, which is not fully collimated and thus has a collimation angle, which is half of the aperture angle. The collimation angle may be, for example, 1 or 1.5 degree and the aperture angle thus, for example, 2 or 3 degrees for gap width smaller than 100 xcexcm. For larger gap widths the aperture angle may be smaller than one degree. The use of an illumination beam with such a small aperture angle improves the method, because the risk of a speckle pattern occurring in the printed image is reduced thereby.
The method of the invention may be further characterized in that use is made of a radiation source which emits radiation of at least two discrete wavelengths.
In a conventional proximity printing apparatus a mercury arc discharge lamp is generally used as the radiation source. This lamp emits radiation of different wavelengths, also called different (spectral) lines. Usually, radiation of one of the wavelengths is selected to illuminate the mask and radiation of the other wavelengths is prevented from reaching the mask. The method of the present invention allows simultaneous use of radiation of different wavelengths by adapting the design of the mask to the different wavelengths. Thus, more efficient use can be made of the available radiation.
Preferably the method of the present invention is characterized in that use is made of a mask which comprises areas which differ from each other in that their image information is encoded such that upon illumination of these areas and their surrounding areas with one illumination beam, the sub-images of these areas are formed in planes different from the planes wherein the sub-images of the surrounding area are formed.
This method allows simultaneous printing of different portions of the mask pattern in different image planes, thus in different layers of one device, and is thus very suitable for the manufacture of devices composed of sub-devices arranged at different heights in the device. An example of such a device is a micro-optical-electrical-mechanical system (MOEMS). When designing the diffractive element, or computing the amplitude level and phase level of each image cell, not only the specific lay-out of the device to be manufactured and the illumination wavelength, but also the gap width, or the distance to the image plane, is an input parameter. Thus, it becomes possible to design a diffractive element which has multiple focal planes within a single image field, which is a unique feature not known in the technique of proximity printing or wafer steppers and wafer step-and-scanners. This feature can be used to great advantage for the manufacture of a composed, multiple plane, device on a substrate having a step configuration. The device pattern portions for the different planes can be imaged simultaneously, so that a lot of time can be saved and alignment steps are no longer necessary.
An embodiment of the method, for forming a desired image with a minimum feature size of the order of 3 xcexcm, is characterized in that use is made of a mask comprising image cells having a size of the order of 1 xcexcm, a transmission of the order of either 100% or 0%, introducing a phase shift of either 0xc2x0, 90xc2x0, 180xc2x0 or 270xc2x0, in that the information carrying surface of this mask is arranged at a distance of the order of 50 xcexcm from the selected surface of the layer of sensitive material and in that a mercury arc lamp is used to illuminate the mask.
This embodiment demonstrates that the resolution in proximity printing can be raised to the level which has hitherto been only obtainable with real projection apparatus.
The embodiment is further characterized in that the mask is illuminated by radiation composed of 40% radiation having a wavelength of 365 nm, 20% radiation having a wavelength of 405 nm and 40% radiation having a wavelength of 436 nm.
For radiation of this composition a suitable diffraction element can be designed and if such diffraction element is used, an efficient use will be made of the radiation of a mercury arc lamp. The ratio 40:20:40 for the radiation components holds for the beam supplied by the lamp. Due to absorption in the resist, the effective contribution to the image formation of the wavelength components is 60:15:20.
The invention also relates to an optical mask for use in the method described above. This mask is characterized in that it has the form of a diffractive element and in that the image information is encoded in a two-dimensional array of image cells having dimensions smaller than the minimum feature size of the image to be formed, each image cell having one out of at least two specific transmission levels and one out of at least three phase levels.
It has been proved in practice that a good image can be obtained with such mask, having a limited number of amplitude and phase levels and thus a relatively simple design. A very good image with the above-mentioned minimum feature width of 3 xcexcm can be printed with a mask having four phase levels. If an even better image is required a diffractive element having more than two amplitude levels and more than four phase levels can be designed and used in the method.
A practical embodiment of this mask is characterized in, that image cells with a reduced transmission are coated with a layer having a specific transmission and in that the phase level of each image cell is determined by the thickness of the mask substrate at the position of this cell.
The coating layer may be a layer of chromium, which material is used to full satisfaction in optical lithography. The phase structure may be etched in the mask substrate, for example of quartz, by ion beam technique.
The mask may be further characterized in that it comprises areas which differ from each other in that their image information is encoded such that, upon illumination of these areas and their surrounding areas with one illumination beam, the sub-images of these areas are formed in planes which differ from the planes wherein the sub-images of the surrounding areas are formed.
As explained above, such a mask is very suitable for simultaneously printing image portions in planes at different heights of a stepped substrate.
A practical embodiment of the mask is characterized in, that it comprises image cells having a size of the order of 1 xcexcm, a transmission of the order of either 100% or 0% introducing a phase shift of either 0xc2x0, 90xc2x0, 180xc2x0 or 270xc2x0.
The invention also relates to an apparatus for proximity printing, the apparatus comprising in this order:
a radiation source emitting radiation of at least one predetermined wavelength;
a mask holder for holding a mask which comprises image information and is at least partly transparent to the source radiation, and
a substrate holder for holding a substrate provided with a layer of material which is sensitive to the radiation from the source. This apparatus is characterized in that the mask has the form of a diffractive element wherein the image information is encoded in a two-dimensional array of image cells having dimensions which are smaller than the minimum image feature size, each image cell having one out of at least two specific transmission levels and one out of at least three phase levels.
Preferably the apparatus is characterized in that the radiation source is configured to emit a radiation beam having an aperture angle of the order of a few degrees.
The image quality can be improved with such a, substantially collimated, beam.
The apparatus may be further characterized in that source radiation of at least two discrete wavelengths is incident on the plane of the mask.
Efficient use is then made of the available source radiation.
A practical embodiment of the apparatus is characterized in that the distance between the image information carrying surface of the mask and the surface of the sensitive layer is of the order of 50 xcexcm.
With such a distance, or gap width, an image resolution of 3 xcexcm can be obtained. If a higher resolution is required, for example, 1.5-2 xcexcm, the gap width can be decreased, for example, to 25 xcexcm.
The apparatus may be further characterized in that the radiation source emits radiation composed of 40% radiation having a wavelength 365 nm, 20% radiation having a wavelength 405 nm and 40% radiation having a wavelength 436 nm.
In such an apparatus, a conventional mercury arc lamp is used for the radiation source.
The invention also relates to a method of manufacturing a device in at least one layer on a substrate, the method comprising the steps of:
forming an image, comprising features corresponding to device features to be configured in said layer, on a radiation-sensitive layer provided on said layer, and
removing material from, or adding material to, areas of said layer which are delineated by the image formed in the sensitive layer. This apparatus is characterized in, that the method as described above is used for forming the image.
Devices which can be manufactured by means of this method are liquid crystalline display devices, customer ICs, printed circuit boards etc.
If the manufacturing method is further characterized in that use is made of a mask comprising areas which differ from each other in that their image information is encoded such that, upon illumination of these areas and their surrounding areas with one illumination beam, the sub-images of these areas are formed in planes which differ from the planes wherein the sub-images of the surrounding areas are formed, it is very suitable for manufacturing a device composed of sub-devices at different levels.
Examples of such devices are micro-optical-electrical systems and optical telecommunication devices comprising a diode laser, or a detector, and an optical fibre and possibly a lens between the fibre and the diode laser, or the detector.
These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiments described hereinafter.